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NEW ORLEANS (CBS Houston/AP) — BP has been going to the site of the Deepwater Horizon disaster in the Gulf of Mexico with the Coast Guard to see if any more oil is seeping through the capped containment dome, CBS News is reporting.

Thursday will be the fifth day BP has gone to the site where an April 2010 explosion killed 11 oil rig workers and dumped 7 million gallons of oil into the Gulf.

CBS News learned that a new oil sheen that came from the Macondo well underneath the Deepwater Horizon oil rig was seen this past September, nearly 50 miles off the Louisiana coast.

BP claimed the leaks were plugged after a remote operation in October but non-profit environmental group “On Wings of Care” has spotted slicks and oil sheens since.

“It was impossible to miss this large slick, located within a mile of the site of the DWH incident,” the group’s website stated in a Nov. 9 post. “It is quite a bit larger than known natural seeps within 20 miles of this vicinity.”

The group added, “This large slick rivals the largest of natural seeps we’ve seen and documented in the Gulf.”

Rep. Ed Markey, D-Mass., says BP is stonewalling once again by not turning over evidence of possible Deepwater oil leaks.

“My concern is that substantial amounts of oil could still be leaking from the wreckage,” Markey, who helped to lead the 2010 BP investigation, told CBS News. “Back in 2010, I said BP was either lying or incompetent. Well, it turns out they were both. This is the same crime scene, and the American public today is entitled to the same information that BP was lying about in 2010 so that we can understand the full dimension of the additional environmental damage.”

BP well site leaders Robert Kaluza and Donald Vidrine, along with former BP vice president of exploration for the Gulf David Rainey, pleaded not guilty last month to criminal charges stemming from the deadly explosion and the company’s response to the massive 2010 spill in the Gulf.

Kaluza and Vidrine are charged with manslaughter in the deaths of 11 rig workers. They are accused of disregarding abnormally high pressure readings that should have been glaring indications of trouble just before the blowout of BP’s Macondo well.

Rainey was charged separately with concealing information from Congress about the amount of oil that was leaking from the well.

A trial for Kaluza and Vidrine is scheduled to start on Feb. 4, while Rainey has a Jan. 28 trial date. Both dates could be postponed given the complexity of the cases.

BP announced in early November that it will plead guilty to manslaughter, obstruction of Congress and other charges and pay a record $4.5 billion in penalties to resolve a Justice Department probe of the disaster.